a. Field of the Invention
The present invention relates to electrostatic printing.
B. Description of the Prior Art
Various printing methods are known and have been used. Among them, the electrostatic printing method is known as a peculiar type of printing technique.
There are some important technical differences between the conventional printing method and the electrostatic printing method. In the former, printing ink can be selectively adhered to the surface of a printing master according to a pattern of unevenness formed on the surface or a patterned distribution of difference in affinity for solvent.
The inked surface of the master is then contacted under pressure with a sheet of printing paper so as to print it. In the latter, mechanical or physical inking to the master is not used and, instead, printing ink i.e. toner is electrostatically adhered to the surface of the master. The toner adhered to the master is then transferred to a sheet of paper to print it.
As to printing characteristics, the conventional printing method is featured by the fact that the adhesion of printing ink to the printing master is so good and stable that high speed and large quantity printing may be allowed. However it has the drawback that the printing ink may adhere to undesired parts or articles so as to form ink stains. On the contrary, the electrostatic printing method is unsuitable for high speed printing because the adhesion of toner to the master is so unstable that it cannot endure the severe printing conditions required for high speed printing. But, since the method does not use common printing ink, there occurs almost no problem of dirt and stain. For this reason, electrostatic printing is called "clean printing" in contrast with the conventional printing processes and has been expected to be employed more widely. Nevertheless, until now, its practical use has been limited to a narrow field of application. This is because the electrostatic printing method is behind the conventional one in the sharpness of printings and the number of sheets which can be produced from one and same printing master.
Typically, a printing master for electrostatic printing is made by either forming dielectric image on a conductive support or forming a conductive image on a dielectric support. To form the image on the support, two alternative processes may be employed. One is to apply an image-wise pattern of dielectric or conductive lacquer onto the support. In another process, the support is coated with photosensitive lacquer and then subjected to an image-wise exposure. Thereafter, the exposed portion or unexposed portion is cut out by an etching technique.
When such a master, for example, a master having a dielectric image portion is used, the most common electrostatic printing process comprises the steps of charging the image portion with a static charge so as to form a corresponding static image, developing the static image with charged toner of opposite polarity to that of the static image and transferring the toner image to a transfer sheet, and a cyclic repeat of the steps.
The printed matters produced by this electrostatic printing process lack adequate sharpness of print. Furthermore, the durability of the printing master is not so good. There is thus room for improvement and development in the electrostatic printing. The poor durability of the electrostatic printing master is attributable to the fact that the uneven surface by which an image is formed, is easily damaged by mechanical friction during the printing process. The abrasion of the image surface will cause irregular charging. High resolving power cannot be attained by an image-wise pattern of unevenness. Therefore it is technically difficult to produce printings having a high resolving power by using such a printing master the image surface of which is formed by a pattern of unevenness. Further, with the image formed by a pattern of unevenness, it is difficult to obtain a halftone image or a gradational image.
In the electrostatic printing method with the use of the printing master as described above, the step of cleaning is essential. The transferred image obtained by the process without cleaning step is inferior to that obtained by the process including cleaning step in respect of quality, in particular, of sharpness and resolving power. Without a cleaning step, the adjustment of the developing condition also becomes very difficult. The cleaning step is inevitably necessary for the practical use of the electrostatic printing method. In the electrostatic printing machines hitherto known, the developing agent recovered from the cleaning station has been disposed as waste. Otherwise, the developing agent has been gathered at a definite recovery place and thereafter has been circulated to the developing station for reuse through a regenerator or the like. The recovery and circulation of developing agent have various difficulties. To overcome the difficulties, there have been made various attempts and devices.
At any rate, the known electrostatic printing apparatus necessitates a cleaning device and a developing device separately provided. As a result, the overall size of the apparatus has necessarily become large.